In the field of spin resonance techniques, such as Nuclear Magnetic Resonance (NMR) and Muon Spin Relaxation (\(\mu \) SR), the theory of open quantum systems plays a role in the interpretation of many results when either the dynamics of the magnetic fields or of the atomic nuclei themselves introduce a time dependence in the Hamiltonian. Such dynamical processes play an important role in loss of coherence and thus apparent decay of magnetisation, a key phenomenon in both solid state NMR [1]}, [2]} and \(\mu \) SR [3]}, [4]}. In these fields it's common to treat such problems with approximations that work only when the dynamical processes are far removed from the time scales on which the system's evolution takes place, either by being much slower or much faster. The intermediate regime, known as “spectral diffusion” [5]}, [6]}, can not in general be described with a simple closed form expression.
